Heretofore, electric motor controllers and electric motor control systems to control electric motors have been known. By using a method such, for example, as PWM wave voltage driving, this kind controller controls an electric motor through power conversion means for converting an inputted DC power into an AC power and outputting the AC power. For example, PTL 1 discloses a control technique in which, when an electric motor revolves at low rate with a large torque, the carrier frequency of a carrier signal is switched to a low frequency to prevent the breaking of switching means of the power conversion means attributable to heat. By switching the carrier frequency, it is possible to prevent a current from concentrating in a particular phase, and thereby to suppress increases in the heat and the damage on the switching means.